As a practical matter, that doesn't work when the power imbalance is so great that one side doesn't have a realistic way to negotiate the contract.

I.e. if the best job that someone can negotiate is one where they are being exploited or abused, then that doesn't make the exploitation/abuse any more just.

The trouble is that most people will become homeless if they don't get a job. As long as there is a steady rate of unemployment, there are people that can't meaningfully decline whatever conditions are in their employment contract.

And that's what this pretty good system doesn't account for: the inherent coercion of a fundamentally unjust system

Fair contracts need negotiating power on both sides.